Access networks are known of the kind in which a central office, usually located at a providers premises, is connected by optical fibre to a number of optical network units (ONUs) usually located at a residential premises or business premises. To reduce costs these networks are often passive, called optical PON networks, usually meaning that the routing devices between the central office CO and the ONUs do not require any power or light sources. This reduces infrastructure costs greatly and means all optical sources are located at the providers premises which increases safety. The fibres may be of the order of a few km in length, with the main limitation in a passive network being optical transmission losses.
In a simple arrangement, one wavelength is used for downstream signals and time division multiplexing (TDM) is used to allocate a portion of the signal to each ONU. A different wavelength may be used for upstream signals. A simple passive splitter can then be used to send the correct portion of signal to each ONU.
In a refinement it is also known to use wavelength division multiplexing (WDM) whereby optical signals at more than one wavelength are sent out by the CO, with each ONU being allocated one wavelength (although they could be allocated more than one). Each signal can then be modulated with information to be sent to an ONU. To route the correct wavelength signal to the correct ONU, a passive distribution node is provided which taps into the optical fibre from the CO. The function of the node is to separate out the wavelengths and send them to the correct ONU's. Using more than one wavelength increases the capacity of the network compared with simple TDM networks with a single wavelength.
Many different forms of modulation can be used, although one known network that has been proposed uses differential phase shift keying (DPSK) to encode information on each optical signal. In this scheme, information is encoded on an optical signal by changing or modulating the phase of the signal. The information can then be recovered from the signal by looking at the phase of the signal, and in the particular case of DPSK comparing the phase of a given “bit” of the signal with that of a preceding but. This modulation scheme is advantageous because an inherent property of DPSK is that it suffers little from cross gain modulation when semiconductor optical amplifiers are in the network.
To decode the modulated signals it has been proposed to provide at each ONU a phase sensitive demodulator, typically in the form of a Mach Zender type interferometer. In a simple implementation this comprises a delay line interferometer in front of a photodiode. Since DPSK encodes a binary signal by changing the phase of the signal over time, the delay line allows a bit of signal to be compared to an earlier (delayed) bit of signal using the interferometer. The output of the interfermoter will be intensity modulated and can be read using the photodiode to show how the phase changes from bit to bit.
The applicant has appreciated that the provision of a line delay interfermoter at each ONU is costly. Problems also arise due to the effects of chromatic dispersion between the distribution node and the ONU's.
Whilst this can be overcome with the use of dispersion compensating fibre that again adds to the cost.